Conventionally, various studies have been conducted to reduce noise of a diesel engine (especially noise caused by engine knocks, simply referred to as a “knocking sound” below). For example, JP2012-036798A discloses an art for calculating, as a target value of a time difference occurring between combustion pressure waves generated by a plurality of fuel injections, a time difference with which a pressure level in a high frequency range can be lowered by interference between the combustion pressure waves, and controlling a time interval of the plurality of fuel injections based on this target value. This art aims to reduce the knocking sound by controlling the time interval of the fuel injections to lower a frequency component of pressure inside a cylinder (in-cylinder pressure) to a specific frequency range (2.8 to 3.5 kHz). Note that a “combustion pressure wave” is a pressure wave generated by in-cylinder pressure sharply rising due to combustion in an engine, and corresponds to a time differentiated waveform of the in-cylinder pressure.
Meanwhile, the knocking sound which occurs in the engine has a transfer characteristic of a structural system of the engine, particularly a characteristic corresponding to a resonance frequency of the structural system of the engine. Specifically, the knocking sound tends to become louder in a frequency range including the resonance frequency of the structural system of the engine (a frequency range having a certain bandwidth formed by a combination of resonances between components provided on a main transmission path of the engine, referred as a “resonance frequency range” below). Although a structural system of an engine generally has a plurality of resonance frequency ranges, the art of JP2012-036798A is only capable of reducing the knocking sound in a specific frequency range of 2.8 to 3.5 kHz and cannot appropriately reduce the respective knocking sounds corresponding to the plurality of resonance frequency ranges.
Here, the knocking sound is a characteristic corresponding to, in addition to the resonance occurring in the structural system of the engine described above, an in-cylinder pressure level equivalent to a combustion exciting force (generally referred to as “CPL” or “Cylinder Pressure Level” and indicates a high frequency energy obtained by Fourier-transforming an in-cylinder pressure waveform with a combustion exciting force index). This CPL depends on a heat generation rate indicating a combustion mode in the cylinder, a waveform of this heat generation rate changes under an influence of environmental conditions, such as temperature and pressure, and the knocking sound receives an influence from the mode of such a waveform of the heat generation rate. Therefore, in order to appropriately reduce the knocking sound, it is desirable to set the time interval of the plurality of fuel injections based on a timing at which the heat generation rate, reflecting the influence of the environmental conditions, reaches the highest value (peak).
In view of this point, for example, JP2016-217215A discloses an art for reducing a knocking sound corresponding to a resonance frequency of a structural system of an engine. In this art, a time interval of a plurality of fuel injections is controlled so that valley portions of a waveform indicating a frequency characteristic of a combustion pressure wave generated by a plurality of combustions fall within the respective plurality of resonance frequency ranges of the structural system of the engine. Thus, the knocking sounds corresponding to the respectively resonance frequency ranges are reduced.
Hereinafter, the fuel injection control performed to reduce the knocking sound corresponding to the specific frequency of the engine (typically, the resonance frequency of the structural system) as described in JP2016-217215A is suitably referred to as “frequency control.”
Within a low load range of the diesel engine, since the combustion noise level becomes higher than mechanical noise, traveling noise, intake and exhaust sounds, etc., the knocking sound becomes more noticeable. Although details will be described later, according to research conducted by the present inventors, it was found that performing the plurality of fuel injections before a main injection to cause continuous combustions in the cylinder results in lowering the combustion noise level. However, if an ignition environment degrades due to a decrease in an oxygen concentration, etc., it becomes difficult to perform such continuous combustions and, as a result, the knocking sound cannot appropriately be reduced.